**Core Concept**
Pulmonary embolism (PE) is a life-threatening condition characterized by the obstruction of a pulmonary artery by a blood clot (thrombus) or other material. The source of these clots is often a deep vein thrombosis (DVT) in the legs, which can break loose and travel to the lungs via the bloodstream.

**Why the Correct Answer is Right**
The most common source of pulmonary embolism is a deep vein thrombosis in the lower extremities, particularly in the femoral or popliteal veins. This occurs due to venous stasis, hypercoagulability, and endothelial injury, which are often seen in patients who are immobilized, have a history of cancer, or have other underlying medical conditions. The clots formed in these veins can then dislodge and travel to the lungs, where they can cause significant morbidity and mortality.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the source of pulmonary embolism is not typically a cardiac source, such as a myocardial infarction or cardiac arrhythmia. While these conditions can increase the risk of pulmonary embolism, they are not the primary source of the clots.

**Option B:** This option is incorrect because the source of pulmonary embolism is not typically a gastrointestinal source, such as a bleeding ulcer or bowel infarction. While these conditions can increase the risk of pulmonary embolism, they are not the primary source of the clots.

**Option D:** This option is incorrect because the source of pulmonary embolism is not typically an upper extremity source, such as a subclavian vein thrombosis. While this condition can increase the risk of pulmonary embolism, it is not the most common source.

**Clinical Pearl / High-Yield Fact**
The "well's score" is a useful tool for predicting the risk of pulmonary embolism in patients. It takes into account clinical factors such as age, sex, and symptoms, as well as laboratory findings such as D-dimer levels.

**Correct Answer: C. Deep vein thrombosis in the lower extremities.**